Various devices have been designed and manufactured over time and which are is useful when used in an earthen profile to determine or calculate hydraulic gradients. These hydraulic gradients have been employed to determine the direction of water movement and to estimate water flux using unsaturated hydraulic conductivity. As should be understood, the movement of water in an unsaturated earthen zone is important for engineering studies, hazardous waste site monitoring, recharge studies and irrigation management practices. For example, if the moisture potential of soil can be accurately monitored, irrigation can be controlled to optimize the rate of plant growth.
One type of instrument used heretofore for measuring soil moisture potential is the tensiometer. A conventional tensiometer comprises a sealed tube defining a chamber which is normally completely filled with water; a hollow porous tip on one end of the tube; and a vacuum gauge connected to the water chamber. The porous tip is inserted in the soil and establishes hydraulic contact between the water in the tube and the moisture in the soil surrounding the tip. Relatively dry soil tends to withdraw water from the tube through the porous tip. However, since the tube is sealed, only a minute amount of water is actually withdrawn. Accordingly, the water in the tube is placed under tension by this effect of the dry soil, thus creating a measurable sub-atmospheric pressure in the tube. Higher moisture contents in the soil produce correspondingly less vacuum in the tube, and completely saturated soils register substantially zero vacuum or atmospheric pressure.
Typical tensiometer constructions provide a tube or column of water which extends from the porous tip to above grade. It will be apparent that the deeper the porous tip is buried, the longer the column of liquid above it will become.
Air presence in the water reservoir during tensiometric measurement is undesirable. In this regard, air can enter the reservoir by diffusing through the porous tip. More commonly, dissolved air present in water that enters the vessel comes out of solution in the reduced pressure environment of the tensiometer. In this situation, if left unchecked, the entire tensiometer would eventually become filled with air. This entrapped air will increase the time required to reach pressure equilibrium because large volumes of water must move through the porous tip to effect the mass transfer of air through the tip. Thus, in order to obtain accurate measurements of relative soil moisture, the air is desirably purged from the tensiometer reservoir and replaced with water.
Accordingly, manually operated de-airing systems have been designed for conventional tensiometers. However, such designs are inadequate for use with tensiometers monitoring soil moisture potential deep within below-grade earthen soil. Furthermore, one of the most costly aspects of using tensiometers in the field is the periodic maintenance schedule needed to routinely purge the air from the reservoir.
Accordingly, there remains a need for a method of monitoring soil moisture potential deep within below-grade earthen soil, and to devices which facilitate such measurements. Although a principal motivation for this invention arose from concerns associated with deep soil use of tensiometers, the artisan will recognize other uses of the invention which is only intended to be limited by the accompanying claims appropriately interpreted in accordance with the Doctrine of Equivalents.